Registration with PHP + MySQL

Hey,
I did this tutorial from this site:
http://www.kirupa.com/developer/actionscript/authentication.htm

It tells me how I can let a flash movie check if people are registered.
I was wondering how I could add a function where people can register, so their username + password is stored in the MySQL database (instead of static users that were inserted by me, I want people to be able to add their own username + password)